1. Technical Field
The present invention relates generally to apparatus and methods for the receipt of digital data, and more particularly to apparatus and methods for verifying the operation of digital components which communicate over a bus network wherein the identity of the transmitting device together with data encoded in the transmitted communication are used to convert the communication into an understandable format, such as for example, an engineering notation format.
2. Background of the Art
Many modern electronic systems are composed of digital subsystems in communication over one or more buses for the exchange of digital information. These digital subsystems may be adapted to receive, compute and transmit various operating parameters within the system. The automobile, for example, utilizes various sensing means which measure among other things air temperature, engine speed, and atmospheric pressure, and then transmits this data in digital form to a central computer wherein the data is processed and then transmitted to various receiving units at the automobile engine to control the proper fuel/air mixture entering the automobile carburetor.
On a much wider scale, the modern jet aircraft requires the interchange of numerous operating parameters, such as for example, air speed, altitude, and heading, which are relayed to one or more computing units to be processed and then transmitted to other subsystems within the aircraft. Information exchanged within the aircraft is handled by individual control and information systems dedicated to performing specific tasks onboard the aircraft, such as for example, the air data system comprising an air data computer which receives data from various sensors around the aircraft, and then transmits the processed data via a digital bus network to other receiving units interfaced with the air data bus.
These control and information systems form integrated information systems for safe operation of the aircraft, interfacing not only with each other, but with the flight crew operating the aircraft. Each system includes a transmitting source and one or more receivers, also referred to as line replaceable units (LRU's), the transmitter and receivers interconnected by a common bus for the transfer of the digital data. The data is transferred in the form of encoded words which contain data defining various operating parameters of the aircraft, e.g., air speed, altitude, each word containing an encoded label to assist in identifying the data to the receiving LRU's. Sometimes it is desirable to access these systems for test purposes in order to read the data transferred therein; therefore conventional test sets have been developed which may be interfaced to specific system buses for this purpose. Often however, the same label may be used in different systems to identify different operating parameters transmitted therein; and, therefore decoding of the communicated words by the test set for display in an easily understandable format is impossible using label data only.
The multibit digital communication word commonly used in avionic systems is specified in the Mark 33 Digital Information Transfer System (DITS) published by Aeronautcal Radio, Inc., of Annapolis, Md., as ARINC specification No. 429-7, January 1983, and referred to herein as the ARINC 429 word. A typical binary coded decimal ARINC word, illustrated in FIG. 1, comprises 32 bits, including label data bits 1 through 8, source/destination (SDI) bits 9 and 10, data field bits 11 through 29, sign status matrix (SSM) bits 30 and 31, and a parity bit 32. The function of the various bit groups comprising the 32 bit ARINC word are as follows: the label bits identify the particular operating parameter described by the 32 bit word, for example altitude or air speed; bits 11 through 28 represent the numerical value of that operating parameter, for example if altitude represents the operating parameter then bits 11 through 29 represent the numerical value of that altitude. source/destination index bits 9 and 10 may be utilized to differentiate between two or more LRU's reading the same label; however it should be noted that some binary words cannot have an SDI because more than eighteen bits (bits 11 through 29) are needed for proper resolution of the data field. The remaining bits provide secondary information which will be discussed in further detail hereinafter.
Encoding and decoding of ARINC words under the correct label is accomplished by reference to the ARINC specification. Referring now to FIG. 2 there is shown a portion of the ARINC specification listing the label code in octal format, the source equipment code in hexadecimal format, and the operating parameters represented thereby. It should be appreciated that proper identification of the operating parameters under the ARINC 429-7 specification requires both the source equipment identification code as well as the label code. Referring particularly to label code 034, it can be seen that label code 034 together with source equipment code 02 identifies the operating parameter VOR/ILS frequency represented in units of hertz, whereas label code 034 together with source equipment code 06 identifies a barometric correction represented in units of millibars. Therefore it can be seen that one label code may represent two entirely different operating parameters depending upon the particular transmitting source and system bus. Since the label code is the only portion of the ARINC word which identifies the particular operating parameter transmitted over a system bus, conventional test sets were programmed to receive and display data from only one source or an assumed source. When data from another system utilizing a different transmitting source needed to be analyzed, another test set programmed to receive and display data from that transmitting source was used.
Apparatus for transmitting and receiving data over a system bus utilizing digital signals formatted in standardized multibit words for testing the exchange of digital information within the system, was disclosed in an application by T. Jackson et al entitled "Method and Apparatus for Testing Systems That Communicate Over Digital Buses by Transmitting and Receiving Signals in the Form of Standardized Multibit Binary Encoded Words", Ser. No. 227,371, filed in the United States Patent and Trademark Office on Jan. 22, 1981, now U.S. Pat. No. 4,393,498, the contents of which are incorporated herein by reference in their entirety. The test set disclosed in the Jackson application was directed towards formulating, transmitting and receiving inter-LRU digital communications in the form of communication words, and then displaying these words to a human operator in an understandable form, such as engineering unit format; an engineering unit comprising a decimal number as opposed to binary 1's and 0's which must be converted to another form for rapid understanding by a human operator. The inter-LRU digital communication word used in the aforesaid Jackson patent was defined by the ARINC 429-3 specification, published Dec. 15, 1979. Under the ARINC 429-3 specification, each label was assigned only one operating parameter. Therefore the label data contained in the ARINC word was sufficient to completely identify the operating parameter therein. Due to subsequent changes in the ARINC specification beginning with ARINC specification 429-4, label data was utilized to define one or more operating parameters requiring the user to specify to the test set manufacturers the assignment of one operating parameter to a label. Unfortunately, these dedicated test sets were usable only with those system LRU's which were programmed in accordance with the label assignments of the manufacturers.
The data signals transmitted over the bus network between LRU's are in the form of voltage differentials which in accordance with ARINC specifications comprise bipolar signals of plus or minus 5 volts, +5 volts representing logical 1 and -5 volts representing logical 0. Due to the location of the transmitting and receiving units throughout the aircraft, the bus lines used to transmit these digital signals are susceptible to the pickup of unwanted extraneous electrical signals (noise) from other electrical components in the aircraft, such as from generators motors, etc., which tend to distort and obscure these bipolar electrical signals, and if severe enough can cause the transmission of erroneous data. Conventional methods for reducing noise pickup includes shielding the signal transmission cables as well as utilizing a shielded twisted pair so that the common noise picked up by the lines is eliminated.
The frequency at which the digital data is transmitted over the bus network may be at separate discrete levels. The electronic circuitry receiving these transmissions for transfer to other digital subsystems must be synchonized to the frequency of the transmission source. Under the ARINC specification, data transmission must be at one of two frequencies, either a low speed transmission rate of 12.5 kilohertz (KHz) or a high speed transmission rate of 100 KHz. In conventional apparatus it is necessary that the operator determine the data bus transmission rate and manually select the corresponding frequency so that the data will be properly interpreted at the test set unit. If the operator is unable to determine the transmission frequency, the data will be received in an unintelligible form, if at all.
Therefore what is needed is an apparatus and method which can be utilized to receive, identify, and convert digital data transmitted over a system bus into an understandable format by utilizing both label and source equipment identifiers to electronically define and present the operating parameter in engineering units. There is also needed a method and apparatus for reducing the unwanted noise introduced into the the digital data during transmission between the various LRU's. In addition there is also needed an apparatus and method for automatically determining the frequency of digital data transmission and then timing the associated circuitry to receive that data.